Comparison of two types of synchronization of unidirectionally coupled external-cavity semiconductor lasers

We study numerically the synchronization regimes of optical-feedback-induced chaos in unidirectionally coupled semiconductor lasers. Depending on the operating conditions, the receiver laser intensity, IR(t), synchronizes with the intensity of the optically injected field, IT(t-tc), where tc is the flight time from the transmitter laser to the receiver laser, or synchronizes with IT(t-tc+t), where t is the is the external-cavity round-trip time. In the latter case, the intensity produced by the receiver anticipates the injected intensity IT(t-tc) by an anticipation time equal to t. We find that synchronization with a lag time tc-t is much more sensitive to frequency detuning and noise than synchronization with a lag time tc. Moreover, we also find that though synchronization with a lag time tc can be obtained using either an external-cavity or an open-loop receiver, the synchronization quality is always better when an external-cavity receiver is used.